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What knowledge is needed to build a small aircraft?
It is my dream to make a basic flyable small scale aircraft (the size of a model aircraft). What is the basic knowledge needed to be able get started in the first place?
In particular, I would like to be able to design and build aircraft that can actually fly, while understanding why it all works. I imagine that there is a need to study fluid mechanics, the structures of aircraft, and certain mechanical and electrical systems.
It would be really helpful if you can enlighten me, and help jump start the learning process. In particular, suggestions about learning resources would be appreciated.
P.S. I have at least first-year university physics and I am always learning more.

Related: book recommendations for basic aerodynamics

@fooot So the first step is to get a good understanding of aerodynamics?

That's a big part of the design aspect. At model aircraft scale the structure will not be as critical unless you are really trying to optimize space or weight. And then you have the specifics or RC aircraft systems but we've decided those are mostly off topic for this site.

I'm not saying the book recommendations totally answer your question here but it's a good place to start.

You can start with a flying model kit to learn to fly. Start making changes and then see what is actually the required amount of knowledge around aircraft design principles (which may not be so important). Using servos, radio-control and engine/motor will likely be important parts, more than advanced aerodynamics. Anyway, books here and instructable here.

One way is trial and error. That is how the early pioneers learned it, and building a few models is a good way to learn the basic stuff, too. That would be

Aerodynamics (what forces keep the aircraft in the air)
Flight mechanics (how to control it and make it fly straight by itself)
Structural design (how to make it light and stiff)
Systems (what equipment is needed to operate it)

My next recommendation might sound boring, but reading the federal regulations for light aircraft will cover all those aspects and more. In many cases you will wonder what that particular regulation tries to accomplish: This is your chance to learn! Post a question here and don't give up until you have fully understood why that particular regulation has been added. Yes, this will take a while, but building an aircraft all by yourself is no mean feat.
Last, join the local chapter of the Experimental Aircraft Association or whatever organisation supports experimental aircraft builders in your country.

Aerodynamics are important, obviously. Know the formulas for lift and drag and thrust reqd. This will help you to design the aircraft.
Also, you will want to read up about airframe maintenance, it will have some useful stuff about building aircraft from the common materials and will also give you an idea of how to cover it with whatever material you decide to go with.


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How can I get the ADS-B GNSS Antenna Offset and Position Offset?
I'm reading the FAA's Advisory Circular AC No:20-165 appendix 1 point 28, which talks about GNSS Antenna Offset and Position Offset.
How would I receive this information from an aircraft? Is it transmitted?

If the aircraft is equipped with ADS-B, then yes, this is transmitted in an Airborne Position Message, type code 20 to 22. Are you asking how you physically receive it (ADS-B receiver) or how to decode it?

What I ultimately need to know is where relevant to the fuselage the ADS-B GPS position is (GNSS Antenna). I'm hoping that this information tells where the reading is taken from so I can correctly position the aircraft.

The antenna is typically in-line with the fuselage, but may be placed anywhere along its length. If you are trying to get the offset of the GPS antenna from some fixed point on the aircraft then it isn't possible, antenna positions are not standardized with respect to a fixed point on the aircraft. I think most software assumes the GPS position is at the center (0,0,0) of the aircraft. How accurate are you trying to be?

I'm building 3d models of aircraft that I place on a map, now if the aircraft is at a gate and I receive a ground position (I'm also interested in airborne position too) I need to position the airplane with some degree of accuracy as it may end up in the wrong place as I'm unable to determine where the GPS position is relevant to (let's say) the sky bridge.

You may have a difficult problem there. You may need to assume that if the aircraft is within 1 length of the gate (and stopped) that it is at the gate.

The aircraft length and width category plus the antenna offset should give you what you need. These fields were specifically added to address your kind of usage. Aircraft equipped with ADS-B version 2 (on 1090 MHz) will transmit the info. Earlier versions don't.

How do I get that information? Is there some example or documentation you can point me to? Also is this transmitted if so how often?

The fields are transmitted when the aircraft has ADS-B protocol version 2 (assuming you are looking at 1090MHz) and is on the ground.
They are specifically added to the ADS-B MOPS (minimum operational performance standards) to allow for accurately depicting aircraft on the airport surface.
I don't know by heart which message type it is in, which bits are used and what the encoding is.
Best is to obtain a copy of the MOPS; RTCA DO260B or EUROCAE ED102A (they are essentially the same, get the cheapest).

For ground surveillance would I also need ADS-B Out for Airport Surface Surveillance (ADS-B APT, ED-163 / DO-321)? What information is different in both documents?

The ADS-B MOPS explains the messsage structure and defines tests for ADS-B transmitters and receivers. The ADS-B APT contain the requirements for applying ADS-B for airport surveillance. If you build something for air traffic control or airport operations you will need to get ED-163 as well.

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Is it common to change the type of engine during major overhauls?
Is it common to change out the type of engine during major overhauls of old aircraft?  For example, replacing a 285 hp engine with a 310 hp.  Would this invalidate the performance specifications found in the aircraft's pilots operating manual?

Not common but not unheard of. Overhauls are the major avenue by which older aircraft variants receive upgrades, effectively becoming newer variants (or functionally equivalent to them). Engine upgrades can be one of the easier changes to make, assuming the new engine was designed to be a "drop-in replacement" (mounts and fuel/electrical connections identical, so you just bolt it on possibly with a token change to the housing). Other changes between variants can be much more in-depth, like stretching the fuselage or altering wing length and profile, which are not typically done to aircraft that have already entered service.
In GA, this tends to be more common, as many of the airframes in the GA fleet are decades old (thanks to strict maintenance requirements keeping them serviceable much longer than the expected useful life of a car), and so the benefit of more modern technology of all kinds, including in the engines, tends to justify the costs. Major airlines very rarely do this kind of thing, especially with engines, as jet engines are ridiculously expensive, and when the engine's reached the end of its useful service life, the airframe usually isn't far behind. It's sometimes done to really big, really expensive aircraft like 747s, and the military has done a few "service life extension programs" that involve replacing engines on venerable airframes like B-52s and C-130s.
As far as invalidating ratings, well yes, the original aircraft specs in the POM would no longer apply after an up-engine. The aircraft, as a unit, must be re-rated both theoretically and empirically by an FAA inspector to determine its new specs, and whether all the components of the aircraft can handle them safely (a more powerful engine does not necessarily mean the aircraft can be rated for a higher "do not exceed" speed, if the limiting factor wasn't the engine but the airframe's ability to take the stress). The easiest types of upgrades to get approved are ones that turn the aircraft into a more powerful factory variant of the airframe; for instance, dropping a 235 engine into what used to be a Cherokee 180. With a few other tweaks to bring the entire aircraft up to the specs of a 235, it can be evaluated as if it were one.

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I'm thinking of building a balloon-launched glider, is this idea plausible?
So, I know nothing about aviation. I'm a student studying maths and computing and I feel like building a high altitude glider controlled by a Raspberry Pi and Arduino as a personal project.
First things first, I'll be launching using a weather balloon or something to reach a given altitude (references to any information on glide ratios as you get to higher altitudes would be much appreciated). Once it's high enough the balloon will be deflated and released, the glider will then be dropped and start its descent.
Assume I've control of the glider in the form of waypoints I can plot on a map to be put into the onboard computer beforehand (or from signals sent from a computer). How complex an autopilot would be required to follow a set of coordinates (pre-calculated so they fit the descent pattern) and then release a parachute for a safe landing?
I was thinking of purchasing a basic glider structure as to have the maneuverability from the professionally built wings and stuff, so construction shouldn't be a problem. The main concern is how plausible a project like this would be considering I'll be building the flight computer myself and I know basically nothing about flight control.
Any other material you have on flight control, flight computers, personal projects like this would also be much appreciated.

The first project I heard doing this were a couple of Canadian students who launched a glider via weather balloon that sent GPS coordinates back via radio so they could retrieve it. They lost it somewhere in the mountains. This was back in the early 2000s. Probably 2002. Their flight could have taken place in the 90s. The most well known successful attempt is probably this: http://www.youtube.com/watch?v=rpBnurznFio but make sure you get clearence from your government if you plan to release that high

This idea is entirely plausible and has been tried before, and even successfully. But I guess you have already read through all the pages which you can find via Google.
Several things are to be considered:

It is really cold up there (see below), and the ascent will take long enough that you need to heat the batteries and to remove grease which will likely freeze solid up there. It would be a shame if the onboard camera cannot document the flight because its battery was too cold.
Due to the low air density, flight speed will be high, so your sailplane will likely experience flutter if you try to use a regular RC glider. A more sturdy configuration which is less optimized for high glide ratios might serve you better.
Winds can be very high at altitude, especially between 10 and 16 km height. You need to factor wind drift into your planning. Try to launch on a calm day (calm all the way up, that is), or the plane might not reach the waypoints.

Since lower temperature means a lower Reynolds number, viscous effects will be higher at altitude than on the ground. This means that your glider's drag for a given lift will be higher at altitude. How much depends a lot on the choice of wing airfoil. If you go with an existing model, pick one which flies well over a wide speed range. A reasonable glide ratio for a model at altitude would be between 10 to 15, so it will glide 10 to 15 km for each km of altitude.
The maximum altitude in which subsonic flight is possible depends on the wing loading (among other things), so try to keep masses down.

There is a rule in the FAA part 101 regulations that has been interpreted to mean that you cannot launch anything from a high altitude balloon...
   "(b) No person operating any moored balloon, kite, unmanned rocket,
   or unmanned free balloon may allow an object to be dropped
   therefrom, if such action creates a hazard to other persons or
   their property."
That seems to be for security, keeping people with bad intentions from launching bombs on rockets, etc. Same with most GPS devices being limited to 20,000 ft. However, it you obtain permission to do it, please post a video!  Good luck.

Not that I don't believe you, but linking to the appropriate part of the FAA regs in addition to quoting them is considered standard practice around here. Oh, and welcome!  Your recent answers indicate an interest in ballooning - that will provide an interesting viewpoint to all the pilots we get here.  :)

Considering that unmanned rockers, by design, allows itself and parts to be dropped, I guess the real important part of that regulation is "if such action creates a hazard". Multi-stage rocketry is perfectly legal. That means that the first part of the rule alone does not disallow you to do things. Rocket launched gliders are also a thing which used to be popular in the past at AMA free-flight events.


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How does the Pseudo-Satellite Zephyr land?
I don't see how the Airbus Pseudo-Satellite Zephyr solar plane can land safely.
It this video, it is "thrown" to take off, but no gear or something similar is visible. Will it just land on its belly?
This video shows, I think, landing footage, but only the runway is visible.

I'm guessing if it's light enough to be carried by (multiple) hand and launched, it can probably be recovered with some sort of elevated arrester or net without too much risk of damage.

@aerobot thanks for the guess, but I am not sure what are these, and from searching the internet, it's not clear.. In any case, I can imagine what you mean, but still, the question remains alive!

It could land on grass without significant damage.  In any case, it's designed to stay aloft for months or years.

In the video I posted it lands on the runway @JohnK. So what you are saying is that it lands on it's belly. That could serve as an answer..

I don't know for sure what the design parameters are so I can't say for sure so I can't post as an answer.

I don't see how it could belly land with the props where they are

@TomMcW you are right, I guess...

It flies so slowly that my guess is that they catch it, like the reverse of launching. It's hard to tell if that video shows it landing or launching.

That's why I said "I think" @TomMcW. Having humans catch it on approach seems a bit of a stretch, since if something goes a bit wrong and one of them gets injured, it might get a too big negative impact in the public face of Ζέφυρος.

@TomMcW Plenty of RC model aircraft belly land, with the props positioned in such a way that they might hit the ground during a belly landing. The weight of the craft is light enough not to cause damage and the forces usually just spin them out of the way. This thing is 25 to 33 meters, so maybe that doesn't apply here...

@gsamaras If you're wondering what I meant by elevated arrester or net, you can check out the ScanEagle's recovery system. It's not exactly what I had in mind, nor even the same class of UAV, but I suspect you can get very creative/flexible about how to recover such light/slow vehicles.

After some power googling...
https://www.airforce-technology.com/projects/zephyr/

Launch and recovery The air vehicle is launched by hand. Recovery is
  by belly landing as the air vehicle has neither landing gear nor
  landing parachutes.

https://www.ainonline.com/aviation-news/defense/2016-07-11/solar-powered-pseudo-satellite-display-farnborough-2016

It is hand-launched by a five-man ground crew, and recovered by belly-landing.

http://en.ruvsa.com/catalog/zephyr/

Launch: hand launch Recovery: autonomous belly landing

It has two-bladed props, so they can be positioned to avoid a ground strike.

If you could see my deleted answer, you'd see that my assumption about a belly landing was correct, still providing sources for it was essential, thanks!

The trick was to google "zephyr belly landing". :)

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How might one start a new logbook for an engine that had one lost?
About 6 months ago in a hangar at a local (KLWT), I saw a Piper PA-30 Twin Comanche and instantly wanted it. When I got home, I did a look up on the Registration (N) number and found out that the aircraft was de-registered in 2005.
I did some asking around the airport the next day and discovered that the owner had passed away. After some more research, I discovered that the airplane had one engine and its prop totaled. It had the propeller and engine replaced, but neither had logbooks and that was why the plane is collecting dust.
I really would like the airplane, because it is actually in good condition, yet it sits in the back of the central hangar and is ignored by everyone. So in a nutshell, the airplane is in good condition with airframe, avionics and logbooks for the right engine and prop. There are just no left prop or engine books.
The current owners (the estate of the deceased pilot) do not seem to even realize that the airplane is still sitting there, so I do not think that they would put much thought into selling the plane and I might be able to pick it up relatively cheaply. The only thing that I seem to need is a logbook. So I was wondering if it would be possible to somehow just start a new log after purchase? The other option is to just buy a new engine with books. Any advice?

One way to get a new engine logbook is to send the engine for a factory overhaul.
Your engine will be regarded as a usable core, and will come back with all tolerances within "as new" limits and a new logbook.

First of all, if the engine hasn’t been run since 2005, it is likely corroded internally and is junk.  Don’t worry about the logs, that engine will never legally fly on a certified aircraft unless it is overhauled and receives a new logbook.  The other engine and the whole airframe may also be corroded beyond saving.
Answering your question more directly: you can buy a new logbook.  Your A&P mechanic must list everything known about the engine from whatever receipts the FBO might have, and he must also verify AD compliance.  An AD is a mandatory Airworthiness Directive issued by the FAA.  The A&P will verify which ADs apply to that airframe/engine and inspect the plane to insure the work has been done.  After the A&P completes his work for an Annual Inspection, the airframe will be legal to fly.
Sitting for 10 years is very bad for engines and airframes.  Be careful going forward.  This plane could be a money pit!

Very, very good point. The OP might offer to buy the plane contingent on it receiving a new SAC, similar to buying a home with the provision that it passes an independent inspection. If you end up having to eat the costs of the inspection, it'll probably run you about 2000-2500 bucks for something thorough enough to give you peace of mind, but that's much cheaper than paying the going rate for a Twin Comanche and then finding out it will never fly again.

For an engine or prop, you can simply start a new log with the appropriate explanation as the first entry.
As a bonus, there's a good chance the airframe log will have the installation date and time so you might come up with TSMOH too (if not total time). If you can determine TSMOH, there is probably little value reduction due to the missing log assuming you can determine the engine/prop is airworthy. (it's a missing airframe log that really devalues an aircraft)
But alas there is airworthiness (the strict FAA meaning of airworthiness) of the engine or prop. You want to know that it meets its type design or properly altered configuration and that it is in a safe condition to operate. So AD compliance does come into question here. If the aircraft went through an annual inspection since the engine/prop replacement, and was declared airworthy, although it is possible that the AD status went with the missing logs, it probably didn't. Typically most shops print out a status report for the aircraft including airframe, engines, props, & appliances and staple the whole report together and place it in the logbook pouch. This happens more often than not so if it's there you're good as long as the inspector signed that any applicable AD was properly dealt with... even  the "Previously Complied With" ones. That list then becomes the complying record.
Most shops use some sort of computerized aid for AD status. Usually they save the aircraft profile and report in memory so they have less work next year. If your lucky, maybe they can reprint and resign the list.
If the airframe logbook has an entry for installing the engine maybe there is a hint of where the engine was overhauled. If so, that overhaul agency should be able to pull up a work order and if your lucky, give you more info like total time and what ADs were complied with at that point, etc.


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What are the names of the forces acting upon the side of a vessel?
People often refer to the four forces in navigation as weight, lift, thrust and drag. Are there separate names for the forces acting upon the side of a craft?

What do you mean by "side"? left and right? Remember that drag and lift are just arbitrary decomposition of the total of aerodynamic forces such that lift is vertical and opposed to weight.

@ManuH I sure wish the industry would come up with a term to differentiate the two definitions of lift. It would avoid much confusion

@ManuH Lift is NOT vertical and opposed to weight. Lift is perpendicular to the airflow.

@Federico does this mean lift and weight have the same direction when the airplane is upside down, i.e. with a negative load factor (e.g. during an aileron roll)?

@ManuH depends. usually no, as you will have an AoA such that the lift will be negative (in the aircraft reference frame) and thus still pointing up.

@Federico That's what I meant: pointing up (frame of reference: aircraft) but negative. I understand that if you keep the aircraft's frame of reference, the direction won't change but the value will go from positive to negative. There is still one point I don't understand: perpendicular to airflow is not the same as perpendicular to aircraft (the difference being linked with AoA)

@ManuH probably is better to pass in chat. but shortly: pointing up (frame of reference: aircraft) no, if it is "negative" it points down in that frame. It is "up" for an external observer. perpendicular to airflow is not the same as perpendicular to aircraft (the difference being linked with AoA) that's correct.

As we engineers are extremely boring people, and given the fact that a name seems not to be available, we generally refer to them simply as "side forces".
Or, at least, I never encountered a different term on any of the textbooks that I have read.

Lateral and sometimes transverse or cross- are also used to qualify these forces.


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What is the typical fatigue life of a helicopter airframe?
For military helicopters such as the Sikorsky UH-60 Black Hawk, what is the typical fatigue life of a helicopter airframe (in cycles and hours) ?

You may want to break this into a few questions (this stack normally only answers one question at a time per, well, question.)  The first Q is probably fine on it's own, the second...  You'll probably need to be mores specific than "any dynamic part" since they all will have different life spans...

Hi Jay, thanks for your suggestions. I welcome answers to both questions herein and I will also create another "question" on its own for question 2.

Typical helicopters do not have cycles or hours limits on the non-dynamic components.
Since they are not pressurised, there is no concept of "cycles" and the fuselage is neither stressed, nor subject to the same kinds of forces that airliners are.
They are inspected on fixed hours cycles and specific types might have areas which are subject to non-destructive testing in accordance with operator, manufacturer or regulatory instructions.  Some, most notably Robinsons, are rebuilt by the manufacturer on hours based cycles and return as new.  However, the fuselage and it's major components are not replaced.  They are inspected and refurbished.
The dynamic components are of course lifed but I see that you have a separate question for that.

Hi Simon, thanks for your inputs. To my understanding, the airframe of a helicopter (or aircraft) usually has a design life which is indicated in hours. So it would be helpful to know this number in hours or cycles for specifically military helicopters such as the Black Hawk?

@Raptor I don't know anything about the UH-60 but no helicopter I am familiar with has a fatigue life, including military.

The Robinson R-22 and R-44 helicopters are life limited  to 2,200 hours or 12 years:

The complete airframe, including rotor systems, drive system, control system, and fuselage, must be overhauled as instructed by Robinson Helicopter Company when any of the following occur:
  1. When the helicopter has been operated for 2200 hours since new or since last overhaul

Hi rdp, thanks for your inputs. However, it would be helpful to know: What sort of airframe fatigue life is expected for military helicopters such as the Black Hawk for instance? That is the specific question I would like to ask. Thank you.

This is true, but it's not a fatigue life.  The fuselage, frames etc and inspected, repaired if needed, and refurbished.  They are not replaced unless needed, and are not lifed.


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Why do jet generators produce a higher frequency power then household electricity?
I know in most homes your local transformer provides a 50Hz or 60Hz power source. So all your hairdryers, dishwashers etc. are built to consume 50Hz or 60Hz power at 110 volts or 220 volts, depending on where you live in the world.  On the aircraft engine generators use a much higher frequency (400Hz). Why is this?

See Adam Davis' answer - "A generator or transformer that can handle a given load is physically smaller and lighter at higher frequencies due to issues with core saturation at lower frequencies."

The other question asks why AC instead of DC (as in cars). Here, the question is specifically about the frequency.

One answer contains the following: It's the same reason 400Hz is used rather than 50Hz or 60Hz - weight. A generator or transformer that can handle a given load is physically smaller and lighter at higher frequencies due to issues with core saturation at lower frequencies. A smaller, lighter core can be used for higher frequencies. That's a dupe

This is a bit speculative, and if I am wrong, I will stand corrected.
Based on my little experience with power plant gas turbines, the frequency depends on the rpm of the turbine and the number of poles of the generator. A power plant turbine runs at 3000 rpm (for 50 Hz) or 3600 rpm (for 60 Hz), with two poles (this is the most reliable design, and works with turbines with rotors up to about 4 m diameter.
An aircraft APU is much smaller, and will run at higher for optimized power output. So, for the 400 Hz, you would have, for example, 12000 rpm and 4 poles (or 24000 rpm and 2 poles).
It already has been mentioned that transformers can be way smaller and lighter for higher frequencies, and it may also be that the higher freqencies of the harmonics cause less interference with other systems of the aircraft.

The speed of the generator is held constant, regardless of the engine's rpm, by the use of a Constant Speed Generator or Integrated Drive Generator.

think your right and according to this site (http://www.400hertz.net/faqs.htm) 'This power limits the RPM of the fastest induction motor to a maximum of 3,600 RPM. The maximum speed of a 400HZ induction motor is 24,000 RPM, approximately seven times faster than is possible with a 60HZ motor. This higher speed and the use of higher quality wire and lamination steel make it possible to produce motors with 10 times the power for the same weight and same size as a 60HZ motor.'

This answer is plausible but wrong.  The APU or engine generators could produce 50 Hz or 60 Hz quite easily if that were what was desired.  There are good reasons why the higher frequency is what's desired, as referenced in the one answer to the (hardly a duplicate) question that's mentioned in the comments to the question, above.  See also, this question.


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What is the difference between IMU (Inertial Measurement Unit) and AHRS (Attitude and Heading Reference System)?
According to online sources like wikipedia the difference belies on the addition of an on-board processing system (I assume it means a microcontroller) which outputs already processed data and not simply the raw measures of the different sensors.
However I would appreciate the take on this of someone who has experience on the subject, a more practical standpoint.

From this website:

Ellipse2-A: Miniature AHRS
Ellipse2-A is a small-sized high performance Attitude and Heading Reference System (AHRS). It provides Roll, Pitch, Heading, and Heave data.
This light-weight sensor (45 grams) includes a MEMS-based Inertial Measurement Unit (IMU) integrating three gyroscopes, three magnetometers, and three accelerometers. Running an extended Kalman filter (EKF), the Ellipse-A provides accurate orientation data in both static and dynamic conditions.

So from this definition, the IMU is the sensor set in six Degrees of Freedom measuring in aircraft axes, and the AHRS provides roll, pitch, heading and altitude in earth axes. The difference is a set of axes transformations that is processed inside the unit.
From the same website:

Ellipse2-N is a small-sized high performance Inertial Navigation System (INS) with integrated GNSS receiver. It provides Roll, Pitch, Heading, Heave, and Navigation data.

So the difference between AHRS and INS is that the latter provides additional lat/lon data based on GPS co-ordinates which it obtains from a built-in receiver.
The classic 70s Inertial Navigation System in aircraft used very stable gyroscopes plus accelerometers to sense changes in 6-DoF in aircraft axes. These changes were integrated into positions changes and added to the reference position where the system was initialised. IRS systems pre-date the use of microprocessors, it used to be done by analog integrator circuits and other very clever analog stuff on printed circuit boards. However, the use of integrated signals means error accumulation, like sailors experienced when they integrated the knots velocity of their ropes into navigation position, and then crashed into the west coast of Australia due to accumulated position error.
So nowadays, not only are the spinning gyroscopes replaced by Ring Laser Gyro's, the position errors can be nulled by comparing the results with the GPS data. GPS data cannot be used directly for autopilot control due to the time lag in the signals. The inertia sensors provide results with a high frequency response, and integration errors can be nulled from the GPS and magnetometer signals.


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What is the exact meaning of "attitude", does it include translational movement?
Reading documentation it is clear to me that "attitude" means the study of orientation of a body. However sometimes it seems to be used loosely for the motion tracking of a body (both translational and rotational, not rotational only).
Is it a mistake or a common abuse of language?

Attitude defines the logitudional axis of the plane in relation to the local vertical. What you are refering to is the Flight Path Vector, which is a the result of alle forces acting on the plane.

Your assumption is correct, attitude is orientation in space and it excludes translation.
Object orientation in space can be defined using multiple systems. The one used in aviation uses three angles:

heading or yaw (around the yaw axis)
pitch (around the lateral axis)
roll or bank (around the longitudinal axis).

Source.
Attitude is usually understood as orientation relative to the horizontal plane, that is only two angles:

pitch
roll/bank.

In the common definition heading is not taken into account. For example, in no wind condition, "level flight" is usually obtained by maintaining a constant attitude, that is constant pitch and roll angles. We'll find this sense in the expression flying by attitude, nose-up attitude or unusual attitude.
Attitude is visualized using an attitude indicator (aka artificial horizon):

Attitude indicator, indicating 5° right roll and 10° down pitch
In some applications attitude may include heading as well, and thus becomes a synonym for orientation in space using the three angles:

heading/yaw
pitch
roll/bank.

This use is more often found in flight dynamics and astronautics than in aviation. In this sense attitude and speed help determine the velocity vector. More on that in attitude reference frames.
Heading reference must be specified. In aviation heading reference can be geographic or magnetic North and has its own instrument, the horizontal situation indicator or directional gyro (or the compass as a backup instrument):

Directional gyro, indicating a heading of 113°. Source
The three angles can be combined into a single instrument, this is often the case for head up displays:

Source.
3-angles attitude can be quantified as a quaternion or a matrix of angles, usually Euler's angles (actually Tait–Bryan's angles).
It can be visualized using a spherical attitude indicator (more at Space Exploration):

Orbiter attitude indicator, source

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Who closes an airline's IFR flight plan and when?
ATA (Air Transport Association of America) defines the various flight phases (ATA Spec 2300, flight phases). The last phase is called Flight Close and is described as "The Flight Close phase begins when the crew initiates a message to the flight-following authorities that the aircraft is secure [...]".
In commercial airline operations, who closes the IFR flight plan? The flight plan is usually filed by the dispatchers, I guess. I have never heard a pilot actually close the flight plan before leaving the aircraft...

When landing at a controlled aerodrome, air traffic control will automatically close the flightplan once the flight has safely arrived. The pilot does not have to do anything in this case. The majority of commercial flights take place at controlled aerodromes, which is probably what makes you think that pilots never close their flightplans.
If landing at an uncontrolled field (that is, one without a control tower), however, the pilot will either close the flightplan by contacting a nearby ATC station on the radio before landing, or via telephone after landing.
This applies equally to IFR and VFR flightplans, to big passenger jets and small one-person GA aircraft. If the destination airport — any airport of any size — has an operating control tower at the time of arrival, they will close your flight plan. If the airport does not have a control tower, or the tower is closed at the time, you have to contact another ATS unit to have it closed.
This is actually very logical when you think of it. When you are on a flightplan, you are automatically provided with alerting service. Meaning that if you do not arrive on time, people will start looking for you. If you are landing at an aerodrome with a control tower, the tower is responsible for your alerting service. They have your ETA (based on your FPL EET and ATD) and will initiate a search and rescue operation if you are late. If flying to an uncontrolled aerodrome, some other ATS unit (for example an area-wide flight information service) is responsible for your alerting service, so they are the ones who need to initiate search and rescue if you do not arrive on time.
Now think of this: if landing somewhere with a tower, the tower obviously knows when you land (they clear you to land, after all, and their job is to look at the runway). Thus, the tower knows that you have landed safely and that alerting service can be cancelled.
However, if landing somewhere without a tower, where another unit (not on-site) is responsible for making sure you land safely, how would they know that you have landed? The only way to be sure is for you to call them up on the radio or telephone to let them know that you have safely arrived. They will then file the arrival time to note that you have arrived safely and no search and rescue needs to be initiated.
And this is essentially what "closing a flightplan" is - it actually just means that your arrival time is sent to whichever unit is responsible for your alerting service. It's not that your flightplan is actually "closed" or "deleted" from some system, it just means that your arrival time is filed so that everyone knows the flight has landed.

So, whether they close the flight plan automatically is dependent on where you fly to rather than whether you're an airline or not?  I didn't understand.

@user6035379 That's right. It doesn't matter if you're flying a big commercial plane or a small single engine private plane. If you land at a controlled aerodrome, ATC will close your flightplan automatically. If landing at an uncontrolled field, you have to close it manually.

Really?  By 'aerodrome' do you mean like JFK?  What if you go to other controlled airports; something smaller than an aerodrome.  Do they still close the flight plan for a small single engine private plane too?

@user6035379 If the airport—any airport of any size—has an operating control tower at the time, they will close your flight plan. If the airport does not have a control tower, or the tower is closed at the time, you have to close your flight plan yourself.

@user6035379 Yes, really. I am not trying to mislead you here. You should look up the definition of an aerodrome.

@J.Hougaard said you can contact ATC on the radio before landing. At many non-towered airports in the U.S., you can contact ATC by radio after landing. It depends on the airport, your radio (transmitter strength and antenna quality), and, sometimes, your location on the field (because of transmission obstructions).

If you're vfr but in contact with ATC will they close it for you or do you have to ask?

@TomMcW Same answer. If you land at a controlled airport, it happens automatically.

@TomMcW If you are on a VFR flight plan, the tower has no way of knowing that. You must ask to cancel it with them or after you land by phone. The exception would be if you are flying into an area where a TFR is in effect and flight plans are required. e.g. the outer ring of Presidential visit TFR.

@JScarry Wrong. If you are on a VFR flightplan [to a controlled airfield] the tower absolutely knows this. It is extremely important because of alerting service. The tower will also receive AFTN messages related to the FPL (DEP, DLA etc.)

@J.Hougaard If that is the case, whey have I gotten phone calls when returning to a towered airport from a TFR and been slow to close my flight plan? According to the AIM, 5−1−14. Closing VFR/DVFR Flight Plans
A pilot is responsible for ensuring that his/her VFR or
DVFR flight plan is canceled. You should close your
flight plan with the nearest FSS, or if one is not
available, you may request any ATC facility to relay
your cancellation to the FSS. Control towers do not
automatically close VFR or DVFR flight plans since
they do not know if a particular VFR aircraft is on a
flight plan.

@J.Hougaard US ATC does not have access to VFR/DVFR flight plans, only IFR. It may be different in other countries.

@user6035379 the terminology is confusing, but essentially the opposite of what this answer describes is what happens: when you drop off the radar scope the system automatically removes your flight plan from the system unless the controller takes positive action to preserve it. For flights landing at towered airports they don't do this. For flights landing at untowered airports they do, and if you haven't called to let them know you've safely landed they initiate search-and-rescue.

In the interest of completeness...
expeditedescent's answer correctly describes the effects of the flight-plan closure system. But at least in the USA, the actual mechanics are different.
I had confusion about this as well. The terminology used (by the FAA and others) is that controllers will close a flight plan "automatically" if the aircraft lands at a towered airport, and if the flight lands at an untowered airport the closure is not "automatic."
In fact the inverse is true. Whenever a flight drops off of radar it will go into a "coast" status where the system will keep pushing it along for a few moments based on its last known groundspeed (coasting) and the flight number will show up in the "coast list" on the controller's scope.
If the aircraft's transponder is recognized again within a minute or so, the flight will be taken from coast status back to active status. This might happen if someone was doing a touch-and-go, or made a missed approach, or if they were flying just at the right way to be tangential to the radar site for a moment and thereby dropped from coverage.
If the transponder is not recognized again within the right amount of time, the flight will be dropped from the coast list and a message will be sent to the main flight data processing computer to remove the flight plan from the system. This is what happens automatically, at both towered and untowered airports, unless the controller takes a positive action to prevent it from happening (which they will do, for IFR flights arriving at untowered airports).
The controller will preserve the flight plan in the system, and if they are using flight progress strips they will preserve the strip, until they hear from a trusted source that the aircraft is safely on the ground. Usually this will be the pilot, but it could be airline operations or even the FBO or county sheriff in the case of a general-aviation IFR flight. Only once they know the aircraft has landed will they let it be removed from the system. If they don't get that confirmation they will initiate search-and-rescue operations.
There is another option: the pilot can elect to cancel their flight plan in the air before they reach the airport and proceed under visual flight rules. Generally airlines' procedures will not permit them to do this, but general-aviation pilots will often cancel in the air.

Hi @randomhead. Nice to have another controller around :) Obviously procedures in the US are different from Scandinavia where I work. In our system, the radar track and the flightplan data are two separate entities, so removing one does not affect the other. If a track drops from the radar, we still have the flightplan and need to check that the flight arrives within the calculated ETA, otherwise SAR is initiated. To me this makes more sense than what you describe, since a track could drop from radar at any arbitrary position, not just at its destination (e.g. crash enroute)

@expeditedescent: To me this makes more sense than what you describe—welcome to the FAA!

@expeditedescent from what I read here it appears to me, that the USAnian ATC system (in regard to systems, technology, controllers support, and so on) is like it used to be in Europe 15 or 20 years ago


text:
How does the fan in the jet engine suck in air while the engine is stationary?
This question is related to the fans and compressors in jet engines.
(Please Note: I know how a propeller driven airplane works. The propeller has the shape of the airfoil and when this propeller moves through air, a pressure difference is created between the front and the back of the propeller. This causes the airplane to move forward)
Most of the fans that I have encountered till now merely recirculate the air in an enclosed area (for example: the ceiling fan recirculates the air already present in the room)
I am comfortable with the idea that fans recirculate air when they rotate. But my intuition is failing me when I think of fans (exhaust fans) that suck air out of an enclosed area.
The exhaust fan sucks out air from an enclosed area (ex: room). The fan in a jet engine sucks in air from outside into a enclosed area.
I would appreciate it if someone could explain how these fans manage to suck-in/suck-out air. Does it have anything to do with the shape/orientation of the blades of the fan?

neither the room nor (and especially!) the jet engine is an enclosed airtight area.

The air being pushed out the front of the fan has to come from somewhere. Put your hand behind your desk fan, you will feel a slight breeze there as air rushes in to fill the low-pressure void created by the spinning blades. The air flow won't feel as strong as it does in front of the fan because it's pulling from a large area, while the column of air in front of the blades is much more concentrated (for a while). As you move your hand away from the front of the fan you'll find an area where the air pressure feels about the same as it does right behind the blades.

@FreeMan thank you for your answer. The desk fan explanation was quite good. If I find myself near a desk fan, I will check if there is a breeze behind the fan.

@vrn That would certainly be easier than trying to feel a breeze above a ceiling fan! :-D

The answer in all cases is the fan accelerates air, creating a low-pressure area, and more air moves in to fill the low pressure.
Consider a ceiling fan.  As it runs, the air in the disk of the fan is accelerated downwards.  This means that the space where that air used to be is now at lower pressure, since some of the air has been removed from it. Air below the fan is moving downwards, away from the fan, so it's difficult for it to fill the low-pressure zone. So the air that moves in will come from the top and the sides.
If you put the fan inside a tube (a duct), the air can't come in from the sides, so it must come from above.  You now have something conceptually similar to a vertically mounted jet engine, except that it's much simpler (actual jet engines have multiple fans) and it's powered by a little electric motor, rather than by injecting and burning fuel.
If you want a pump, the fan doesn't care if you put in a false ceiling that divides the space above the fan from the space below it, with the duct and the fan as the only connection between the two.  Since no air can get into the area above the fan, the fan will pump the air out of that zone.  Eventually, the pressure in above the fan will get so low that air the fan is accelerating downwards will start to flow back up into the evacuated zone.  This is why any particular fan will only lower the pressure by a certain amount.

thanks for being the first person to answer this question. I have marked this as the accepted answer. The concept is very clear now. Thanks again

It's very simple. It's the orientation of the blades and the direction in which they're turning. You notice that the blades are placed in a way that they push the air also causing a vacuum from the back obviously. Take ur fan for example and observe the position of the blades and the direction they rotate in. Use ur visualizing skills and see that as the blades rotate in that direction they push that invisible air causing a suction from behind.


text:
How long does it take for an FAA licence to be delivered?
Would anyone have an idea how how long it takes for a licence to be posted out.
The airmen search tells me my ATP processing was completed at some point last week. So the licence has been finalised.
Do they post the licences straight away and do they use Fedex or DHL, normal or priority?
I live in Australia. And need to present this licence at an interview in a weeks time which is why i ask. Are there means of expediting or tracking the licence in the mail?
Between the flight school and the fsdo losing my paperwork and forgeting signatures and corrections being sent to the wrong FSDO offices, its taken me about 5 months to get this licence processed. Ive missed out on some job offers already because the companies would not accept temporary licences. Id rather not miss out on this one.
If anyone has a clue, it would be most appreciated. I know fedex says its about 5 working days economy mail and UPS says 7 working days economy mail. But i have never delt with either.
Thank you kindly

Your transit times seem like they assume continental United States, I'm guessing if they are sending something to you overseas it would take a few days more.

Thank you for your response. I used their online calculators, from Oklahoma to Melbourne AU. I dont suppose there is anything i can do about it except wait and hope for the best. Im getting quite the work out checking the mail box exery 2 seconds in any case lol.But thank you again Ron

According to the most recent Information it takes minimal of 30 days and maximum of 60 days and above.

You should have received a temporary certificate after your checkride

The FAA has, at least in the recent past, been backlogged with some forms and certificate issuance has been one of those items. My private pilot certificate took nearly 3 months to arrive (hence, my temporary certificate almost expired). I had to make several calls to my DPE (Designated Pilot Examiner), who then reached out to the FAA in Oklahoma City to see what was wrong. It turned out a computer glitch had erased my knowledge test report. After several times around, this was fixed and my certificate showed up within several weeks. In the USA they always send it via First Class Mail (uncertified, no tracking number). Last August (2016) my new certificate showing my instrument rating showed up just a month or so after the checkride, so much faster. If you have a question, don't hesitate to reach out to your DPE - they are very helpful and can get things moving again if they're stuck. Also, make sure you keep all test reports and other documents until you receive your certificate in case they somehow get lost in the system. And when you get your new certificate - don't forget to sign the back or it is not legally valid!
Regarding your interview - your temporary airman certificate is valid for all purposes until the expiration date. You can bring this to an interview as it is as valid as the plastic card. I don't know if a company is allowed to reject you because you only have a temporary certificate while your plastic one is being prepared - I'd look into this and maybe ask an attorney as that doesn't seem like it would be legal to me since the certificate is valid.
Cheers!

Thank you Pugz.  I think with the employer its not a choice. But the regulator demands the licence to be verified with the FAA. And the FAA said to me they will only accept verification requests from the regulatory body only, in my case CASA, not from the employer or the airmen. However CASA will not send a verification request until they see the permanent licence card. God knows why. They can easily verify without it. Mind you the longer they hold on to your application here the more money you pay. Its 180 dollars reprocessing fee each time the application goes past 14days unfinalized.

Hmmm sounds like typical government extortion... that must be a foreign thing. It doesn't work that way here.

I found the FAA quite a breath of fresh air to deal with. My american freinds give me a look of horror when i say that. Its worse out here. Lol.

In recent times, it’s taken about 45-60 days from issuance of the temporary airman certificate.  The FAA does list what license applications are being processed on their website under airmen Certification so you can get a general idea of when you will receive your license in the mail.  Again, as mentioned, your temporary airmen certificate is valid for all practical and legal purposes until the permanent one arrives.

The temp certificate is useful only in the USA, for those of us needing validations it's useless. The FAA use the cheapest mail they can find therefore sending a license by mail can require up to 60 days which is about how long it took in the 1920s.
One would have thought by now they could have taken an online payment to use FEDEX which would reduce this to just 3 days.


text:
For flight planning must the forecast weather be above minima one hour before, after or at ETA? (EU-OPS)
Citing EU-OPS:

CAT.OP.MPA.185 Planning minima for IFR flights — aeroplanes
(a) Planning minima for a take-off alternate aerodrome

The operator shall only select an aerodrome as a take-off alternate aerodrome when the appropriate weather reports and/or
    forecasts indicate that, during a period commencing one hour before
    and ending one hour after the estimated time of arrival at the
    aerodrome, the weather conditions will be at or above the applicable
    landing minima specified in accordance with CAT.OP.MPA.110. The
    ceiling shall be taken into account when the only approach operations
    available are non-precision approaches (NPA) and/or circling
    operations. Any limitation related to OEI operations shall be taken
    into account.

(b) Planning minima for a destination aerodrome other than an isolated destination aerodrome

The operator shall only select the
    destination aerodrome when:

(1) the appropriate weather reports and/or
      forecasts indicate that, during a period commencing one hour before
      and ending one hour after the estimated time of arrival at the
      aerodrome, the weather conditions will be at or above the applicable
      planning minima as follows:

(i) RVR/visibility (VIS) specified in accordance with CAT.OP.MPA.110; and
(ii) for an NPA or a circling operation, the ceiling at or above MDH;

or
(2) two destination alternate aerodromes are selected.

(c) Planning minima for a destination alternate aerodrome, isolated aerodrome, fuel en-route alternate (fuel ERA) aerodrome, en-route alternate (ERA) aerodrome

The operator shall only select an
    aerodrome for one of these purposes when the appropriate weather
    reports and/or forecasts indicate that, during a period commencing one
    hour before and ending one hour after the estimated time of arrival at
    the aerodrome, the weather conditions will be at or above the planning
    minima in Table 1.

[...]

On the other hand:

CAT.OP.MPA.245 Meteorological conditions — all aircraft
(a) On IFR flights the commander shall only:

(1) commence take-off; or
(2) continue beyond the point from which a revised ATS flight plan
    applies in the event of inflight replanning,

when information is
  available indicating that the expected weather conditions, at the time
  of arrival, at the destination and/or required alternate aerodrome(s)
  are at or above the planning minima.
(b) On IFR flights, the commander shall only continue towards the
  planned destination aerodrome when the latest information available
  indicates that, at the expected time of arrival, the weather
  conditions at the destination, or at least one destination alternate
  aerodrome, are at or above the applicable aerodrome operating minima.
(c) On VFR flights, the commander shall only commence take-off when
  the appropriate weather reports and/or forecasts indicate that the
  meteorological conditions along the part of the route to be flown
  under VFR will, at the appropriate time, be at or above the VFR limits
CAT.OP.MPA.246 Meteorological conditions — aeroplanes
In addition to CAT.OP.MPA.245, on IFR flights with aeroplanes, the commander shall only continue beyond:

(a) the decision point when using the reduced contingency fuel (RCF)
    procedure;
or
(b) the pre-determined point when using the pre-determined point (PDP)
    procedure,

when information is available indicating that the expected
  weather conditions, at the time of arrival, at the destination and/or
  required alternate aerodrome(s) are at or above the applicable
  aerodrome operating minima.

So, should a commander of an operator's flight opt to commence a flight when the forecast at the ETA is above the minima, or the forecast needs to be above the minima for the period from one hour before the ETA till one hour after the ETA?

Weather above planning minima one hour before and after ETA for flight planning and for determining alternate requirements (2 alternates vs 1).
Weather above planning minima at ETA for commencing flight.
Weather above operating minima at ETA for continuing flight.

One way you can think of it: in the cockpit weather at ETA. In the briefing room weather one hour before and after.

Ok, so for example, if in the briefing room, the forecast is "ok" (above minima) for those two hours, and no alternate is selected. In the cockpit, the commander receives word that the forecast is not "enough ok" for those two hours, but it will be "ok" at ETA. He can commence the flight?

(no alternate as per CAT.OP.MPA.180 (b))

That is correct SlumpA- the legal requirement for not planning an alternate was met, therefore the flight can be filed with no alternates designated. After the flight is filed, planning is over. Thus, the 1-hour-before-and-after rule no longer applies. From this point on, the commander simply needs to ensure the weather at ETA is at or above minimums in order to take off, and to continue the flight.


text:
How long does a non-ETOPS flight need to be able to fly in an one-engine-out situation?
Let's say that you're in a 737, mid-cruise in a non-ETOPS situation, and you lose one engine. Are there any FARs governing how long you need to be able to stay airborne? Or is all that's required the ability to continue safely and land at nearest appropriate airport? Are you required by any regulation to be able to fly for an X number of hours before landing?
I couldn't find anything in the FARs about this, only regulations regarding ETOPS and engine out climb. Just want to make sure I'm not missing anything. Thanks!

None that I know of, most FAR's deal with being able to climb out on a single engine if one fails during takeoff (for twins, more engines have different requirements). Most twin engine airliners can fly just fine on a single engine, but probably not at cruise altitude and not for an extended flight. It is unlikely though that the pilot would elect to continue the flight unless the destination was closer than an alternate.

Note that loss of pressurisation is always worse (regarding diversion options) than loss of engine, because loss of pressurisation forces you down to 10,000 ft and that is way less efficient flying—and there is much more terrain that limits your routing there—than at single engine drift-down altitude, which is much higher.

No, nothing addresses that case.
Let's say you're 20 minutes out from your destination, and because it's a clear VFR day you have minimum normal reserves -- roughly an hour of flying (on 2 engines, at medium-to-low altitude).  Those reserves are already prescribed for you by FAR's.  Now you lose the engine.  How long can you fly?  Until you run out of fuel or the other engine quits -- so in all probability, about that same hour, plus or minus.
Different case, you're over at Atlantic heading to Bermuda or the Azores or Iceland or NYC to the Caribbean, or over the Gulf of Mexico -- not ETOPS but a long ways from anywhere.  The fuel on board will be lots more, because you were planning for a much longer flight.  (And again, those fuel requirements are defined elsewhere.)  You now have the added twist of drifting down on one engine to a single-engine ceiling -- you set max continuous thrust on the good motor, and descend at a speed that gives you max range, which probably comes out to a few hundred feet per minute of descent.  Back in the flight planning, the check had to be performed that at the worst case, you could lose the motor, drift down, and continue to reach a suitable airfield without needing more gas than you'd have.  But in something like a 737, the gas you'll have on-board anyway is almost certainly more than sufficient.
But in this latter case, the time you can stay aloft is going to be hours -- way beyond what you had in the first case.
So no, the regulations don't prescribe a time that you have to be able to keep flying after losing one engine.  You DO have to be able to keep flying -- that's important -- and you have to have enough fuel to get to a destination with the one motor shut down.  But in terms of a prescribed time, that isn't given in the FAR's.

To further clarify, if nothing prevents continuing to fly a twin with an engine out, is there any altitude requirement -- e.g. FAR or airworthiness limitation -- to fly at some minimum altitude, perhaps giving possibility to glide to a runway if the second engine fails?

@mins  No, the same rules govern altitude as with everything operating normally. As a matter of fuel economy, the aircraft will tend to remain high until it's time to start down for a landing, but it's not really about being able to glide to a runway in case the other engine quits. Engines on modern jets fail so rarely that you're already in a very rare case if 1 quits; the second motor quitting is a rare^2 case; if you glide to a runway, that's great, but dual engine failure isn't really a trained scenario, the way losing 1 motor is.

@RalphJ: A double engine failure isn't quite a rare^2 case; given that a lot of engine failures are from things that affect both engines equally (fuel exhaustion/starvation/contamination, volcanic ash ingestion), the chance of having the second engine fail given that you've already an engine failure is much greater than the chance of having an engine failure in the first place (because, if one engine fails, there's a good chance that whatever did it will also take out the second engine in a minute or two).


text:
Which models are these aircraft?

Can somebody please identify the second aircraft. I tried reverse search but no use :C.
I'm trying to win a boeing 777 model :D

Will we get part of the 777???? :-)

DC-9 (probably a DC-9-10 but not 100% sure), BAC-111, and Sud Aviation Caravelle, from top to bottom.

It is a DC-9-10. Only the -10 had the wing fences and the different sweep angle at the trailing edge of the wingtip

You could improve this answer by providing other pictures of the aircraft for comparison.


text:
How does an auto-pilot control altitude changes?
When changing the set altitude with an autopilot with speed control engaged how exactly does the control system work...
I can envision one of three (four) methods.

Control throttle and adjust attitude to maintain speed at required descent rate, or
Change attitude to descent slope and adjust throttle to maintain speed, or
Change attitude to maintain descent rate and adjust throttle to maintain speed. or
Some combination of the above.

Follow up question....
When setting the descent rate alone, i.e. no target altitude, is there a difference in control method.

Short answer:  it depends on which mode your flight guidance system is in.  It can be either number 2 or 3, although sometimes even something else entirely.  Whether you're using a target altitude or descent rate, it still all comes down to what mode you're in.  If you're in FPA, the elevator is adjusted to maintain descent slope.  If you're in VS mode, the elevator is adjusted for descent rate.  If you're in FLC/IAS/SPD, the elevator is adjusted for airspeed.  You should reference your pilot's operating handbook as the exact names and engagement methods for these modes varies a little.
When talking about autothrottle and constant speed altitude changes there are two independent inputs to the system, pitch and thrust, and two outputs of the control system, which are vertical speed and airspeed (or flight path angle and airspeed). You either control vertical speed with throttles and airspeed with the elevator, or airspeed with the throttles and vertical speed with the elevator.  It's possible to mix both throttle and elevator for both airspeed and descent rate (as indicated by number 1 and 4 on your list) but every flight guidance system I know of keeps the two separate to avoid confusion.
Advanced flight guidance systems often have three basic altitude change modes: flight level change (FLC), also sometimes called IAS flight guidance, vertical speed mode, and flight path angle mode. There's more on the differences between VS and FLC here.  I believe assume you're talking about the authrottle in airspeed control with the flight guidance in vertical speed mode.
In vertical speed with autothrottle speed control, attitude control and thrust control work somewhat independently .  Your elevator will be set based on the vertical speed control law to target a certain vertical speed, while your throttle will be trimmed based on its own control law to maintain the target airspeed.  So if you need to decrease descent rate but increase airspeed, your throttle will generally increase while your airplane will pitch up.
You mention adjusting to the descent "slope", which would probably refer to flight path angle (FPA) mode. This mode basically tells the autopilot to adjust the elevator so that the flight path is at a certain angle (not the pitch, but the flight path).  This is a common alternative to vertical speed mode, and autothrottle will perform similarly for either mode.  If FPA is available the pilot can select it instead of VS on the guidance panel.  It's less used than VS, and you can read about why you would use one or the other here.
Flight level change (FLC), IAS, or even sometimes called SPD mode, behaves somewhat differently in that the elevator is used to determine your airspeed.  Thrust then determines your vertical speed and usually this is achieved by setting the throttles to climb or idle settings depending on the situation.
Of course, there's lots of what ifs that go into these systems like stall protection, overspeed protection, non-steady-state behavior, autopilot-authrottle interactions, and engine-out behavior.  I won't cover those details here.

WOW.. Thanks for taking the time to write such a complete answer. I had a feeling it was a much more complex answer than a question. :)


text:
Should an emergency field landing be performed with landing gear down or up?
Suppose you are flying an aircraft with retractable gear and have to make an emergency landing on that big "flat" stretch of grass up ahead and have only one shot at it, what's better, do it with wheels down or up.
Obviously, if the ground is flat enough, wheels down may be great and may even let you recover your aircraft basically undamaged. However, hitting a rut or other obstacle with the gear could have some rather unfortunate consequences.
That left me wondering which is better, dropping the gear and hoping for the best, or a controlled belly landing. (Also hoping for the best!)
I also wonder what, if any, instruction is given on this situation during flight training.

I'd say that almost always gear down is better, I'd rather rip the gear off than nose over in that rut or ditch. GA landing gear is remarkably strong.

From a glider point of view : during an outlanding, the only thing between my spine and the rocks hidden in the grass are my pants, a worn-out cushion and 5mm of plastic. I will always chose to add a wheel under all that.

Also, from having landed many times on flat stretches of grass or dirt (generally but not always actual airports), that flat stretch of grass could be perfectly suitable for a gear-down landing and a subsequent takeoff.  Landing with gear up eliminates that possibility.

There are various factors here:

Gear acts as a speed brake, in some emergency situations if you need the added drag as you approach the field (to make the field) a gear down situation may be called for regardless of ground condition. On the flip side if the field is on the edge of gliding distance you may want to keep the gear up to improve glide. This is a judgement call during the emergency.
Gear will provide you limited steering on the ground which for what ever reason you may need. In a gear up situation you are sliding to a stop.
Gear will provide you brakes, if the field is short you are going to want this option.
If something is critically wrong with the plane and you need to do an emergency gear deploy there may be no time during the emergency.

Like anything, it depends and it will be a situational decision with merits to both aspects. You should consult the POH for your air frame as they will most likely have check lists for both scenarios.
For example in a Piper Arrow they offer a checklist of a gear up emergency specifically:

Gear up emergency landing:
In the event a gear up landing is required,
  proceed as follows:
(a) On aircraft equipped with the backup gear
  extender, lock emergency gear lever in "Override Engaged" position
  before airspeed drops to 115 mph to prevent landing gear from
  inadvertently free falling.
(b) Flaps as desired.
(c) Close throttle
  and shut off the master and ignition switches.
(d) Turn the fuel
  selector valve to OFF"
(e) Contact surface at minimum possible airspeed.

Here is what happens in a small plane when the gear collapses on a grass field. A gear up landing is not an automatic hull loss, in many cases they can be repaired. Considering the age and use level of most of the countries GA fleet you may be surprised to find out just how many planes have actually been through a gear up landing.
I was always instructed during my training that ditching gear down (or with a fixed gear) in water was not really viable. There seems to be some info indicating that there is little data to support this. Of course you will find endless debate on the internet about it and some answers that fail to provide sources. Some of the misconceptions may be fueled by what happens when a float-plane digs in on landing.

you say that "you are not going to ditch a fixed gear plane" however there are many cases where pilots have done just that, and the plane did not flip or cartwheel on the surface. In fact one recent one, on Lake Ontario, resulting in both occupants being rescued by boaters in the area, while the plane antennas stayed above the surface thanks to lotsa air in the fuel tanks.  So is this your opinion, or experience, or do you have some aggregated statistics?

I had 2 different flight instructors give me this info. Im not saying it cant be done and Im sure there are successful cases but my flight instruction has always been, runway->green->brown->trees and avoid water in a fixed gear at all costs.

there are lots of flight instructors, with lots of opinions, many of them excellent.  However, those flight instructors have varying experience, and some have extensive knowledge or first hand experience, and some repeat what they have heard, read and believe. One thing anyone can do is to look at the database of accidents and examine SE fixed gear ditching outcomes. Me? If I had the choice of steep granite or shallow warm water near boaters, in a C172 or C172, I would go for the bath.

Here's a report on a Cessna Caravan that ditched in 2013 http://aerossurance.com/news/ntsb-report-2013-c208-ditching/    The C208 is fixed gear, 8500# single engine turbine aircraft.

First, the official guidance is the POH.  After all the manufacturer knows his equipment best.
Second, shearing off wings and gear consumes energy.  The trade off is whether control can be maintained as long with the gear coming off.  On the other hand, at the speed you are touching down at, control (at least aerodynamic) will be lost pretty soon. Discretion on the part of the pilot is a function of his training and his ability to reason through his situation.
With respect to instruction, the standard practice is to cover emergency procedures on an initial aircraft checkout.  My critique sheet for lessons has boxes ticking off items, and off field, water, and other emergency landings, engine out or with power are covered.

A large amount of aviation training is directed toward emergency procedures.
Forced and precautionary landings are an important part of that training.
Most trainee pilots will not be able to undertake their first solo flight until they can demonstrate the skills necessary for this type of emergency.
There are a number of steps involved. The following is a simplification.

Pilots (particularly light aircraft or general aviation pilots) should always be on the lookout for suitable landing locations in the event of an emergency.
If an emergency occurs already knowing your preferred land spot saves valuable time.
Assessment of landing surfaces includes the best type of surface available (flat rocks etc), length, slope, and wind direction.
For example: Even with a fixed undercarriage, water would be better than a surface with large unavoidable rocks or boulders.
The next step is to set your aircraft to fly at the best glide speed (each aircraft has a different best rate of glide speed for particular aircraft total weight = Least height lost vs most ground distance covered).
Navigate towards your chosen landing area using techniques to be at the correct altitudes at points prior to landing. The preferred option is to attempt to complete a normal landing procedure circuit. If the landing is precautionary and you have the ability, undertake a low-level pass to check the quality of the surface.
Communicate your position, problem, intentions, and people on board to the Area or emergency or local radio frequency.
Complete problem-solving checks
Prepare aircraft and passengers for landing, switch off fuel and unnecessary electrical systems. Radio is the last electrical system to be shut down.
Use short field landing techniques (ideal speeds and braking)
Evacuate and move away from the aircraft, once it stops.

As you can see, even abbreviated the process is a detailed exercise, which must be practiced regularly. One cannot obtain a pilot's licence without demonstrating proficiency!!

This doesn't answer the stated question.


text:
How does the pressure change in the Air Cycle Machine?
In this picture:

#3 says:

"Air expanding to ambient pressure turns expansion turbine, is greatly cooled, and drives compressor."

I understand all of that except for the "air expanding to ambient pressure" part.
Ambient means relating to immediate surroundings of something so the pressure can change. I understand that if it decreases in size like at #1 it would get hotter or if it increases in size it would become colder, but how can it be ambient if all the air is expanding to the same size when at #3 the column that the air passes through doesn't change shape to change the pressure?
Basically, how can it be ambient pressure if the column doesn't change size?

Not sure if I understand the question. If we replace "ambient pressure" with something more specific like "15 psi", does the diagram make sense to you or not?

Two things: 1) this is just a schematic diagram, and does not reflect the dimensions of the actual plumbing used in an aircraft, and 2) pressure can change while the 'column' area remains constant, if the flow velocity is adjusted accordingly.

Strictly speaking you're right, the authors used a brief and easy to visualise but slightly inaccurate phrasing. The blue bit is open to the cabin, and at the end of it will have cabin ambient pressure. At the beginning of it the pressure will be slightly higher, but very much lower than before the turbine.
The pipe diametre does not need to change for the pressure to vary. Air streams from high pressure to low pressure, upstream pressure will always be higher than downstream pressure, by an amount to overcome the friction forces.

The underlying principle is explained by the Ideal Gas Law  aka Pv=nrt.
https://en.wikipedia.org/wiki/Ideal_gas_law
When the compressed air, which has been cooled expands to an ambient pressure (like 15 or perhaps less PSI at altitude) the air cools in temperature. It is mixed with cabin air, which creates a warmer mixture, but still cooler than the cabin.
You ask how it can be ambient if the column doesn't change, but the column does change.  The pressure in the intercooler is much higher than ambient, and has been heated by virtue of compression.  In the intercooler it gets cooled, and when then expanded back to "ambient" it further cools, so that it is cooler than ambient.
If it appears that I didn't understand your question, perhaps you could restate the question.

The expansion turbine removes ALOT of heat from the airstream converting it to work turning the turbine which turns the compressor. This accounts for both a pressure drop & a temperature drop


text:
How does the Airbus ECAM / ND transfer work?
On the Airbus A320 a knob on the switching panel says ECAM/ND XFR (Electronic Centralized Aircraft Monitor / Navigation Display Tranfer) and it transfers the either the Engine/Warning Display (E/WD) or the System Display (SD) onto the Navigation Display (ND). The ND gets its information from the Air Data Inertial Reference System (ADIRS) and IT and the ECAM gets its information from the system sensors.
Both ADIRS, IRS and system sensors information get sent to the Display Management Computers (DMC), and I'm just wondering if im correct but when you switch SD or E/WD the DMC when it is processing the data for, in this example the system sensors, it sends the system sensors data to the ND?
And the other way around when you switch the ND to the ECAM. When the DMC is processing the data, in this example ADIRS and IR data, it will send the ADIRS and IR data to the ECAM?
So my question is, is it during the processing the data phase that the DMC sends the data to thd correct display?

Please try to write your questions a bit clearer. When using abbreviations, introduce them first. Try to make your question better readable by using a line of whitespace between paragraphs. And pay attention to grammar. I've tried to clean it up a bit, please review my changes and improve further.

Please can you precise when you talk about PFDU, NDU, EWDU or SDU. It is a little bit harsh to understand what you want to know.

All this depends on the A320 generation. On early models all computations are done by the SDACs and the DMCs, and the switching corresponds to data switching in the DMCs, so the screen will display what it receives.
On later models all screens receive all data and on request they can display what you ask when you do the ECAM/ND switching.
In fact in latest models of A320 the displays switching possibilities are voluntarily limited just for conformity with earliest A320’s


text:
How does one calculate true altitude?
"An aircraft is cruising at FL170. The altitude shown with QNH set is 17,500 feet. The OAT is -10 deg C. What is the true altitude?"
The ISA temperature deviation is 9°C.
My calculation is:
17,500'
 + 9 °C x 4' x 17,500/1000
= 18,130'
The answer provided is 18,100'. I don't understand where my mistake is.

To solve the true altitude, you will need the local barometric pressure, which is not practical.  If you are over water, a radar altimeter might give you the most accurate reading (a reasonable approximation of absolute altitude).  A GPS might come close, but will likely be off no matter where you are, for lots of reasons. You will need a clarified question, or more data.

Apart from knowing a formula or two, this question does give us everything we need to know to answer. The question is clear, and very answerable.

Ok thanks, the answer provided is 18,100' and I have now calculated 18,130' (17,500' + (ISA temp deviation of 9 deg C x 4 deg C x 17.5))?

What pressure reference datum is QNH using in millibars or inches Hg?  That would be needed to evaluate this problem on a flight computer.

@CarloFelicione That reference datum is available here since it can be determined from the difference between the pressure altitude and the indicated altitude with QNH set.

This can be calculated by knowing a few formulas, or temperature relationships related to altitude. Note that all these calculations are an estimate based on standard ISA temperature and altitude relationships. See this page for other examples of this type of question.
Note the following from the question:

FL170 represents a pressure altitude of 17,000 ft
We know that the OAT is -10°C
Indicated altitude is 17,500 ft

Now, to solve for true altitude, we need to know a few additional items. We need to know that:

ISA at the surface is 15°C
Standard lapse rate is -2°C per thousand feet
To find true altitude, the difference from indicated altitude is 4 ft per 1°C deviation from ISA for every 1,000 ft

Knowing all this, we can calculate the following:

ISA at 17,000 ft (see 4 and 5 above)
Deviation from ISA (see 2 and 7 above)
True altitude (see 6 and 8 above)

This all might sound complicated, so lets run through an example problem. Let's assume a flight at FL190, with an OAT of -15°C, on a standard pressure day.
Since ISA at the surface is 15°C, using the standard lapse rate of -2°C per thousand feet, we know that ISA at 19,000 ft will be -23°C.
Given an ISA of -23°C, we see that our OAT of -15°C shows a difference of +8°C above ISA. Follow that step carefully, because the signs can be confusing.
Given that the temperature is ISA+8°C, and knowing 6 above, we can calculate our true altitude relative to pressure altitude:
$A_T$ is our estimated true altitude, in feet
$\Delta_{ISA}$ is the difference above ISA, in °C
$A_I$ is our indicated altitude, in feet
$A_{Ik}$ is our indicated altitude, in thousands of feet
$A_T=(4 \times A_{Ik}  \times \Delta_{ISA}) + A_I$
$A_T=(4 \times 19  \times 8) + 19000$
$A_T=608 + 19000$
$A_T=19608$
In some cases, a question might ask for true altitude based on a pressure altitude, instead of an indicated altitude. In such cases, substitute pressure altitude for indicated altitude in the above formula. Remember, this is all an exercise in estimation which is helpful in reinforcing the relationships between temperature, pressure, and altitude.

Nice way of educating without providing the final answer.

IF YOU USE TEMP LAPSE RATE OF -1.98 C per 1000 feet INSTEAD OF -2 you will get the right answer (temp deviation is 8.66 instead of 9 C ) : true alt = 17500 + 4* 8.66 * 17.5 = 18106 very near to 18100 feet

Thank you for the explanation. How do we know when to use PA or Indicated Altitude in the formula which you have provided(AT=(4×AIk×ΔISA)+AI)? I have noticed some people using Indicated altitude by default and then others use PA by default. So which one is correct?

@Skydemon, The formula that I have provided requires indicated altitude. If working with pressure altitude one must first convert that to indicated. The distinction will typically depend on whether the aircraft is flying above or below the transition altitude, in the flight levels or not. An aircraft flying in the flight levels will be flying reference pressure altitude; and aircraft below the transition level should be flying reference indicated altitude using a QNH or local altimeter setting. Does that answer your question?

@JWalters so if I understand correctly, would we use the Pressure altitude for questions that mention FL and conversely use IA or altitude corrected for pressure error for questions that give the elevation? There is a lot of confusion around which value to use in the formula. Oxford uses only the PA and BGS uses only IA or elevation. Do you have any advice on which value to use for which type of question?

@JWalters for example in this question: You are flying at FL 160. Outside air temperature is -27°C, and the pressure at sea level is 1003hPa. What is the true altitude? Would you use FL160 or altitude corrected for pressure error? The examiner has used IA i.e. 15,730ft

This question has already been thoroughly answered by other users. I just wanted to mention that the non-approximated version of the formula to compute the true altitude is:
$$h_{true} = h + \frac{h}{T_0}\cdot\left(T_\mathrm{OAT}-T_\mathrm{ISA}\right)\tag{1}$$
where the value usually represented by $\frac{4}{1000}$ is in fact $\frac{1}{T_0}=\frac{1}{288.15}\approx0.00347$

First, True Altitude is applicable to a column of air above a point on the sphere. A QNH is for a station, and in ATC that location is arbitrary, and in places such as the UK is one of 20 or so regions. Such QNH data is accurately called a Regional Pressure Setting, and is the lowest forecast pressure for a defined region within an hour. So for survey work, or precision remote sensing, there would need to be an observation at the point below an aircraft, or at the very least an interpolation (modeled) using known values.  In this instance, we don't have that, rather we have a Regional Pressure Setting. Therefore, the resultant figure may be close, but is not the true altitude for the aircraft.
The formula below is from the 1976 International Standard Atmosphere, and reduced by a friend (physicist) who has developed a set of formulas for navigation. The formula provides an accurate answer as it is directly traceable to the 1976ISA, and is not an approximation.
Relationship of true and calibrated (indicated) altitude:
TA= CA + (CA-FE)*(ISADEV)/(273+OAT)
where
TA= True Altitude above sea-level
FE= Field Elevation of station providing the altimeter setting
 CA= Calibrated altitude= Altitude indicated by altimeter when set to the
       altimeter setting, corrected for calibration error.
ISADEV= Average deviation from standard temperature from standard in the air
    column between the station and the aircraft (in C).
OAT= Outside air temperature (at altitude)
Substituting:
ISADEV = (stdISAtemp - (PA * (lapserate)) - differentialfmstdtemp )
TA = 17500 + (17500-0) * (15-17500/1000(2)-(-10)) / (273 - (-10))
TA = 17500 + 17500 * +10 / 283
TA = 17500 + 665.39
TA = 18165


text:
What powers the air-conditioning on commercial planes?
How is the air conditioning system on airliners powered? I expect that the air conditioning is powered by the APU powers, but I wonder if there are other power sources used like the main engines?

Your question is unclear. What stage of operation are you asking about? Large turbine aircraft typically have multiple power sources or systems for air conditioning which will vary depending on ground or flight operation and may also vary depending on altitude.

It sounds like you are thinking of aircraft A/C as if it were the same sort of compressor-driven heat pump affair as on homes.  Not so at all; ample supply of compressed air radically changes how you run A/C.

What powers the airconditioning on most airliners is APU or main engine compressor bleed, which is being supplied at fairly high pressure, say, 80 psi, and temperature, around 3-400F, to the air conditioning packs.  You normally have a choice to select APU or main engine bleed.  You will obviously use APU bleed on the ground before engine start, but also may opt to take off with APU bleed running the air conditioning instead of the engines for slightly better takeoff performance.  On some airplanes, packs on APU bleed can be a requirement for departures in icing conditions because of the performance drain of bleed being used for anti-ice and air conditioning at the same time.
Large jet aircraft (and large turboprop) air conditioning doesn't use a gas vapour cycle like regular air conditioners (some smaller jets and turboprops, that can't produce the bleed flow, as well as piston a/c do however); they get the same result using just the high pressure, high temperature bleed air using a "bootstrap effect", where air is cooled off while still at high pressure, than letting it drop close to ambient pressure after a lot of the heat has been removed, resulting in air that can be very cold.
At the pack, part of the flow gets chilled and part gets bypassed while still hot.  Temperature regulation is by mixing the chilled flow and bypass flow. Electrical power is only required for the control system.
The air being chilled goes through an Air Cycle Machine. An ACM takes high pressure air, drops the temperature using heat exchanger pre-coolers (air-to-air radiators) while the pressure is still high, boosts the pressure and temperature again with a compressor, takes out some of the heat from that pressure boost with more heat exchangers, and you end up with air that is still at high pressure but the temperature is way less than the original bleed.  Then you suddenly let the pressure drop close to ambient by allowing it to expand into a much larger discharge duct.
Air that started off at 80 psi and 400F is now down to 150F while still at 80 psi.  When you let it drop to ambient, the temperature drop from the pressure drop will take the ACM discharge temperature down to just above freezing (theoretically, if you put all the bleed through an ACM, you can get the air down to deep sub-zero temps, but the control system will normally keep the discharge from going below freezing).
The compressor is powered by a turbine in the discharge flow where the expansion temperature drop happens. It's like a big turbocharger unit (the compressor driven by a turbine in the same flow is what constitutes the bootstrap effect).  The final temperature that goes to the cabin is a mix of chilled air and hot air that is bypassed from the ACM by the control system.
You may also be able run the system from a high pressure ground bleed air source (called a "Huffer Cart"), but airlines usually use Huffer Carts for engine starting and instead will run ground air conditioning units that connect directly into the aircraft low pressure ducting (when you see the yellow flex pipe hooked up to the fuselage at the gate, that's a ground low pressure air conditioning source).

This answer provides incorrect information regarding vapor cycle machines on jet aircraft. Many large turbine (jet) aircraft use vapor cycle machines in the air conditioning process.

@JWalters "Many large turbine (jet) aircraft use vapor cycle machines": really? I've only ever seen PACKs (Pneumatic Air Cycle Kits) on jets, I've never heard of one with vapor cycle. Can you give an example?

I'd be interested in an example of a large aircraft using a refrigerant in a closed system, which is what vapour cycle refers to.Even the bleedless 787 uses electric compressors to provide the hot high pressure air in place of bleed. Cooling is still by a bootstrap Air Cycle Machine, compressing air to get it hot, taking away the heat, then letting the pressure drop.  Small corporate airplanes,especially turboprops, may use vapour cycle refrigeration systems because their engines can't produce the bleed volume.  The earlier Citations had to use boots on the wings for deice for the same reason.

The CE-560 (a §25 large transport category) uses a closed vapor cycle machine using freon to supplement the ACM. See also the CE-525 series, the CE-550, etc. I'm just throwing out examples that I am familiar with. Part of the rational with this design is that the vapor cycle machine can produce very effective air conditioning from EPU electrical power alone.

Well those are what I referenced in my comment above.  They use vapour cycle because the engines can't produce the bleed flow. The  Citation V has boots for the same reason, which are a terrible alternative to heated LEs and are only used because there is no alternative.  My post is about airplanes like Regionals and heavies, say 50000 lbs and up.

If you were speaking narrowly of only a subset of large jets then you should edit your post accordingly. The aircraft I referenced are by definition Large Turbine Powered Aircraft. Moreover, the Citation V/Ultra does have wing boots (which are effective) in combination with heated leading edges, but later iterations in that model and similar designs have complete heated leading edges. Likewise some varients do not have the VCM, and only use the ACM, which is fully powered by the engine bleed air. Your assertions that these engines do not produce sufficient bleed air are erroneous.

Although I thought the reference to "airliners" at the top of the post covered it, I added some additional text.

Neither "airline" nor "airliners" are defined terms. Many airlines use piston or turboprop powered aircraft, including the Cessna 402 and 208, Beech 1900, etc.

@JWalters Ok ok...you think too hard lol.

Airliners at cruising altitude need cabin heating, not cooling. Can they use waste heat from the engines, like smaller aircraft or (fossil fueled) cars? Is this worth another Question?

They are using the same hot bleed air, being tapped off the engine compressor (or APU), for either.  As I said, some of the hot bleed air is bypassed around the pack and mixed downstream with cooled air to get the final temperature going to the distribution system.  If cooling the cabin, more is going through the ACM to be chilled, if heating, most is being bypassed.

The running engines power commercial airliner interior climate and environmental control through their built in electrical generators and bleed air systems. The units that supply the interior air in the correct temperature and pressure for passengers are sometimes called PACK as an acronym for Pneumatic Air Cycle Kit.
When the engines are off, the APU (Auxiliary Power Unit) takes over the duties of supplying electricity and bleed air. If an APU is not used, electricity is supplied by a GPU (Ground Power Unit). The climate control is supplied by an outside air conditioning unit temporarily attached to the aircraft.
When the engines are on, neither the APU nor GPU are used nor necessary. The APU is actually turned off when the engines are running. It is an unnecessary drain on fuel.

It's not a normal A/C unit
Aircraft do not use a Freon based compressor vapor-liquid transition heat pump type arrangement the way every home, car, boat and train does.
They use a simpler cycle, because most turbine aircraft have a great abundance of compressed air available, or are obliged to have compressors anyway for pressurization.
The compressed-air A/C cycle
Consider the lowly shop compressor, mainstay of every auto paint shop, auto repair shop, car restorer and metal shop. When you compress air, it gets hot.  Compressing to 135 PSI (10 bar) increases the density of the air by 10 times, which puts all that air's heat in 1/10 the space.  It also puts the air's humidity in 1/10 the space, only possible because hot air holds more water.   Then, the compressor has cooling fins or an intercooler to cool the air.  This reduces the air's ability to hold moisture, and most of the water condenses.
The reverse happens when the compressed air is released (decompressed).  There's too little thermal energy for too much volume of air, so the air comes out cold.  (and dry).  If you've ever used an air tool, you notice they get cold.
Of course, we don't need 10 bar of compression to get this effect; that was just an example of a shop compressor.
In the aircraft
It is the nature of turbines that they compress very large quantities of air for combustion.  It's no trouble to tap the compressor stages to obtain "bleed air" for HVAC uses.
Similar to a shop compressor, an intercooler is used to cool and dry the typically 150C bleed air.  This is is fairly complex but outside air (ram air; or on the ground, blown by a fan) is used as a heat sink. Now you have compressed air at around 3 bar and roughly ambient temp.
When this is allowed to de-compress to cabin pressure, it cools considerably - as low as -20C.  This is too cold for comfort, so hot (non-intercooled) bleed air is mixed in to achieve the desired temperature.


text:
What can I learn before I actually go to flight school (Austria)?
I am currently occupied with a large project which basically eats up all my money but will be done in a year or so. After that, I would like to get my PPL. So I am currently out of money but do have some time left which I would like to use to already study the basics.
Is there any freely available material targeting PPL training? I live in Western Austria if that is of any concern (this is also where I would like to perform any training - LOIH is closest to my location).

Welcome to aviation.SE! This question is about India but it's very similar, it might give you some ideas and you can check out more linked/related questions from it.

Hi. In fact I saw this question when posting but it really was targeted at India and I would say that regulations are probably rather different there. It also mentions the FAA which is US, even if most of the material will be related to EASA as well. I downloaded the PDF from there and I will need to find the material from the other link since the link itself was dead. Do you know of any material which is published by EASA (such as the material published by the FAA)?

Found it, anyone able to update the link from the thread of @voretaq7 to https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/airplane_handbook/media/airplane_flying_handbook.pdf

I've fixed the links, thanks for pointing that out!

Get hold of a simple simulator to familiarize yourself with the basics. I enjoyed Microsoft FSX . It's not intimidating. You can follow along just getting the basics, and you can hear generic pilot talk. It's not going to guarantee you a jump start hours wise, but at least when you get to flight school, you may have heard some of the terms and even messed with certain controls.
Don't expect to skip ahead without serious time investment. A few hours playing on a simulator(emphasis on PLAYING) will help with familiarity and maybe confidence, but be prepared to spend a lot of time listening to your instructor.
Another good thing, though it sounds silly could be to invest in some training videos. I personally enjoyed the courses offered by sporty's pilot shop. It was $200 when I bought it, but I would argue that it paid for itself, since I needed less time paying instructors and renting planes to practice things.
On the other hand, sometimes you hear instructors say they prefer blank slates as opposed to people coming in with bad habits. Maybe be patient and wait until your instruction begins. That way you start off with the habits that your specific school likes to train.
Good luck!

I have trained lots of people from private through CFI, and have not found (in my unscientific sampling) there to be a substantial difference between those who played flight simulator and those who didn't, in terms of private training.  In some cases there may be some time savings on doing procedures in instrument training.  Oddly enough, the biggest differentiator was those who had a spouse/close friend who flew (grin). They at least showed up having spent more time thinking about the previous lesson, and with a gaggle of questions.